MODULE 5: Redox and Electrode Potentials

Question 1

Balancing equations by oxidation numbers:

Answer 1

• Assign oxidation numbers to each species in an equation
• Balance species that have a changed oxidation number
• Balance any remaining atoms

Question 2

Reagents/products likely added to an equation when predicting:

Answer 2

H+, OH- or H2O- usually to balance atoms or charges on each side

Question 3

Two examples of redox titrations:

Answer 3

Fe(2+)/MnO4(-) and I2/S2O3(2-)

Question 4

Procedure for MnO4(-) titration:

Answer 4

• Add KMnO4 to burette
• Add sulfuric acid to conical flask to reduce KMnO4 to MnO4(-)
• Add MnO4(-) in small increments- it’ll decolourise as it is added
• No indicator is needed as the reaction is self indicating, the end point is when a permanent pink colour remains
• Repeat titration until 2 concordant results are obtained

Question 5

Use of MnO4(-) for percentage purity of Fe(II) compounds:

Answer 5

• Prepare 250 cm3 standard solution of impure iron sulphate solution to volumetric flask
• Using a pipette, measure 25 cm3 of the impure solution and to a conical flask, add 10 cm3 sulphuric acid to the solution
• Titrate against MnO4(-) solution
• Calculate moles of MnO4(-) that reacted from mean titre
• Calculate moles of Fe2+ that reacted in 25 cm3
• Multiply by 10 to get moles in the whole 250 cm3 solution
• Calculate mass of unknown sample
• Calculate percentage purity using mass of solution reacted/mass of impure sample x 100

Question 6

Uses of iodine/thiosulfate:

Answer 6

• ClO- concentration in bleach
• Cu2+ concentration in copper (II) compunds
• Cu concentration in copper (II) alloys

Question 7

What indicator is used in an iodine thiosulfate titration?

Answer 7

Starch (colourless to black)

Question 8

Standard electrode potential definition:

Answer 8

The tendency to be reduced and gain electrons compared to hydrogen half cell under standard conditions

Question 9

Measuring a standard electrode potential:

Answer 9

• Half cell must be attached to a hydrogen half cell
• Connected via a salt bridge in each solution to allow the movement of ions (soaked in a solution that wil now react with either reactant)
• Connected by a wire between the electrodes to allow the movement of electrons

Question 10

Metal/metal half-cells:

Answer 10

An electrode made of a metal is placed into a solution of the same metal (but aqueous.) Where the ions come into contact with the electrode, an equilibrium is established:
X 2+ + 2e- X(s)

Question 11

Ion/ion half-cells:

Answer 11

A solution made containing ions of the same species in different oxidation states. An inert electrode of platinum is used to establish this equilibirum:
X3+ + e- –> X2+

Question 12

Meanings of standard electrode potential values:

Answer 12

More positive: more likely to be reduced

More negative: more likely to be oxidised

Question 13

Cell potential equation:

Answer 13

Cell potential = positive electrode standard pot. - negative electrode standard pot.

Question 14

Predicting feasibility of a reaction:

Answer 14

Using standard electrode potentials to calculate the standard cell potential (positive = feasible)

Question 15

Limitations of predicting feasibility of a reaction using standard electrode potentials:

Answer 15

• Activation energy may be too large for the reaction to start, causing a slow rate of reaction/no reaction at all
• Standard electrode potential is measured at 1 mol dm-3, so if the solution is less concentrated it’ll be less feasible, but if it is more concentrated it’ll be more feasible
• Similar applies to other conditions (temperature and pressure)

Question 16

Primary cell:

Answer 16

• Non-rechargeable as the oxidation/reduction reactions are irreversible
• Used in wall clocks and smoke detectors

Question 17

Secondary cell:

Answer 17

• Rechargeable
• Car batteries, radios, torches
• Lithium batteries in phones, laptops

Question 18

Fuel cell:

Answer 18

Uses the energy produced from the reaction of a fuel with oxygen (combustion) to create a voltage
Features:
- Can operate continuously as long as fuel and oxygen are always provided
- Do not have to be recharged